The invention relates to an optoelectronic device and a method for forming an optoelectronic device which may include a photoreceptive array, a photoemissive array, or both. Further, the invention teaches a receptive device and method for forming a receptive device wherein a microlens array exhibits maximum fill factor, good uniform spot-size and intensity, optimum distance from photoreceptive elements and good lens quality. The invention also teaches an emissive device and a method for forming an emissive device wherein the microlens array can satisfy the conditions for collimating, focusing or directing emission from photoemittive elements to specific target.
The optoelectronic array in an imaging integrated circuit (IC) chip is one of the crucial components in defining the sensitivity and resolution of electronic imaging devices such as digital cameras, scanners and printers. Due to complex signal processing requirements associated with each pixel of the optoelectronic or photo sensor array, a significant area surrounding the pixel is covered with metal and signal processing elements which do not convert the impinging light into an electronic signal. The condition wherein light is not optimally focused on photoreceptive elements contributes to a degraded image quality and impairs other performance characteristics. The loss of light to non-converting surface area contributes to loss of fill factor. Fill factor may be improved by coupling a tightly spaced microlens array with the optoelectronic array. To achieve maximum fill factor, the spacing between the microlens elements must be minimized and the spatial relationship between the microlens array and the optoelectronic array should be such that all the light collected by the microlens forms a spot-size which is substantially smaller than the photosensitive area of the photo sensor element at the plane of the optoelectronic array. This enables complete coverage even with less than perfect alignment between lens and optoelectronic array elements.
Some known lens array methods rely on multiple application and removal steps, including application and subsequent removal of an opaque metallic layer (see Popovic et al, U.S. Pat. No. 4,689,291). A common lens array element spacing technique involves etching through a pedestal layer creating a barrier space between microlens elements, then, after applying photo resist (which will serve as a lens element), etching through the photo resist layer. The method is not without several serious drawbacks. Perhaps most limiting is that this method and those similar to it are limited in the closeness of the spacing. This limitation is owing to the tendency of the individual lens elements to fuse together across the barrier space. Secondly, in a method calling for multiple alignment steps, each alignment step adds loss of precision. Loss of precision impairs the manufacturability, making it difficult to repeatably produce a device in which the microlens focuses a spot-size substantially smaller than the photosensitive area at the plane of the optoelectronic array. The closer the spacing of the microlens array, the more crucial alignment becomes, and with more than one alignment step, the possibility increases of misalignment of microlens elements and optoelectronic elements. Such misalignment will result in loss of signal and degraded resolution. What is needed is a method for making optoelectronic devices in which the method has a minimum number of alignment steps and the devices so made have maximum fill factor.
In other applications such as optical interconnects, optoelectronic devices can be useful in collimating, directing and focusing the light emitted from an array of photo emitters onto specific target elements. A method providing control over optical capture and transmission is desired and much sought after, especially where such method provides for the creation of devices exhibiting any of the following: maximum fill factor, uniform spot-size, uniform spot intensity, good lens quality, optimum spacing between the microlens array and the optoelectronic element plane.
Many optical devices require collection and transmission of light. Scanners, digital cameras, and printers all require full capture of light to support output resolution. Any loss of light results in degradation of performance. In other applications such as optical interconnects, control over microarray density and placement directly translates to control over directing light to specific target elements. Thus control over optical capture and transmission is a much sought after device feature.